Method of tempering glass articles, and bath therefor



Patented June 18, 1940 UNHTED STATES PATENT OFFlCE METHOD OF TEMPERINGGLASS ARTICLES, AND BATH THEREFOR No Drawing. Original Serial No.34,107.

application July 31, 1935 Divided and this application June 2, 1937,Serial No. 146,120

8 Claims.

This invention, which is a division of my copending application, SerialNumber 34,107, filed July 31, 1935, relates to glass and moreparticularly to a method of tempering or case hardening glass articles.

Present and pastmethods used for the case hardening of glass articles byfirst heating them to an elevated temperature, then plunging the heatedarticles into a liquid bath comprising oil, waxes, resins, or the like,heated to a lower temperature or subjecting the heated articles to airblasts or any other means for rapid cooling, do not, in general, producea uniform and satisfactory case hardening of the article over its entiresurface without introducing an objectionable or even serious amount ofwarping. The

use of molten metals or alloys as glass tempering media is particularlyliable to cause warpage on account of the great difference in specificgravity between the metal and the glass. Heated oil and like organicmedia are relatively unstable at the temperatures most useful fortempering glass, and because the most useful temperatures are near orabove the flash point of the oil, a fire hazard is introduced into theprocess. Moreover, the carbonization and other efiects'of hightemperatures on oil change the thermal transfer condition between thebath and the heated glass article with the'result that methods employingoils and the like organic media have definite limitations. Past methods,also, in the case of certain glasses have resulted in a lack of chemicalstability of the surface after the chilling operation and the surfaceappearance also at times is seriously affected.

The object of this invention is a method of tempering or case-hardeningglass articles which will produce a uniform and definite degree oftemper without danger of warpage of the articles nor serious effect uponthe surface stability thereof and without introducing any fire hazard.

The above and other objects may be accomplished by employingmy inventionwhich embodies among its features heating and/or quenching the glassarticle in a glass tempering bath composed of a molten inorganic salt orsalts the density or specific gravity of which is substantially the sameas that of the glass whichis to be tempered. 1

Another feature of my invention comprises heating the article which isto be tempered in a bath of molten inorganic salt or salts and chillingthe glass in a bath which will dissolve the constituents of theheating'bath.

Other objects and features will become apparent as thedescription-proceeds.

In practicing my invention I first heat the glass article which is to betempered to a temperature slightly below the softening temperature ofthe glass by immersing it in a bath of molten inorganic salt or mixtureof salts the selection of which depends upon various considerations aswill later appear. After the article has become uniformly heated to thedesired temperature, I remove it from the heating bath and immediatelyplunge it and completely immerse it into a cooling bath which is heatedto a lower temperature depending upon the kind of glass and the degreeof temper desired, and which preferably is com posed of a mixture ofmolten inorganic salts having essential properties, as will appear, butwhich under certain circumstances may comprise heated oil or other priorliquid cooling baths. The chilling step may even comprise subjecting theheated article to an air blast in known manner, the novel feature inthis instance residing in the method of heating the article uniformly toa high temperature without warpage and loss of surface stability.Alternatively, the heating step may comprise heating the article in amufiie or open air furnace to be followed by chilling or quenching thearticle in a bath of molten inorganic salts as above described.

To illustrate the preferred embodiment of my invention, I give thefollowing example of my method as applied to the glass B2 of theSullivan and Taylor Patent 1,304,623, issued May 27, 1919.

This glass has a softening temperature of about 813 C. A slab of thisglass having a width of 1% inches, a thickness of 3 3 inch and a lengthof 7 inches was heated by immersing it for two minutes in a moltenmixture comprising about 67% of sodium sulfate and 33% of potassiumchloride by weight and containing a small amount of tungstic oxide, thelatter of which was added at intervals to keep the bath non-alkaline.The molten bath was maintained at a temperature of 800 C. and had adensity of about 1.9 as compared to about 2.3 for the glass. After beingheated for two minutes, which sufliced to bring the glass to uniformtemperature,

bath comprising about 44% of sodium nitrite and 56% of potassium nitrateby weight heated to 150 C.

Subsequent measurements of the tempered glass slab showed that it hadsuffered no distortion exceeding a very few thousandths of an inch.

the slab was removed and immediately quenched in a molten The mechanicalstrength of the tempered slab was shown by impact tests to be veryappreciably greater than that of other slabs which had the samedimensions and which were tempered by heating in an ordinary atmosphericfurnace and chilled in a bath of 600W oil heated to C. Incidentallyappreciable warpage was encountered in the latter case, due to the slabshaving been heated in an open furnace in accordance with the commonmethod of heating.

As a special feature of the above described method, the following is tobe noted. The chilling bath, which consists of a molten mixture ofpotassium nitrate and sodium nitrite readily dissolves and removes fromthe heated glass article the residual salts which adhere to it when itis removed from the heating bath preparatory to quenching it, therebycleaning the glass and exposing it practically instantaneously to thecooling action of the chilling bath. Briefly, this is explained asfollows: Potassium nitrate melts at approximately 335 C. and willdissolve sodium nitrite at that temperature in all proportions. Theeutectic mixture of these two salts melts at about 141 C. These salts,when molten, will also dissolve at 150 C. a small percentage of sodiumchloride, potassium chloride, sodium sulfate and others, the percentageof which is thus dissolvable increasing with increased temperature.Hence, when the glass which is being treated is removed from the heatingbath with a layer of the salt from the heating bath adhering to it, thislayer of salt becomes dissolved into the potassium nitrate-sodiumnitrite chilling bath.

Another feature of my method which has outstanding importance is thefact that due to some property of molten inorganic salts the temperatureof the chilling bath accurately determines the degree of temper whichwill be obtained. That is to say, the final stress which will beobtained by quenching a given glass heated to a given temperature into amolten salt chilling bath will vary inversely as the temperature of thechilling bath. The degree of temper which is obtained under the sameconditions by using an oil chilling bath is practically independent ofthe temperature of the chilling bath and hence such a bath has a verylimited use.

In tempering articles made of glass other than the glass above referredto, such as, for example, ordinary lime glass which has a softeningtemperature in the neighborhood of 700 (1., it is necessary to employlower temperatures in the heating bath in order not to heat the glassabove its softening point. My method is applicable to all kinds of glassand to a great variety of shapes of ware.

The successful practice of my invention depends largely upon thefollowing considerations:

1. The salt or combination of salts for the heating bath must melt andremain molten at a temperature below that to which the glass articlesmust be heated prior to chilling.

2. The molten salt baths and particularly the heating bath whichoperates at the higher temperature must not attack the glass which isimmersed in it in the time which is required to bring the glass to thedesired temperature.

3. The density of the heating bath should be sufliciently near to thatof the glass to permit the bath to act as a support to prevent the glassarticle from warping.

Other minor requirements, but nevertheless desirable properties of myimproved baths, are as follows:

a. The vapor pressure of the salts employed should be low enough toavoid excessive evaporation at the temperatures employed.

b. The salts which comprise the heating bath and which necessarilyadhere to the glass when it is withdrawn therefrom should not react tooviolently with the medium which is used for chilling.

c. The baths should be non-poisonous and the fumes should benon-injurious to health.

The following is a list of salts and mixtures of salts which I havefound will fulfill the above requirements and which have successfullybeen used as heating baths: sodium chloride, potassium chloride, sodiumchloride and potassium chloride mixed in equal parts byweight, sodiumsulfate and sodium chloride (2 to 1 by weight), sodium bromide,potassium bromide, sodium bromide and potassium bromide mixed, sodiumbromide and sodium sulfate mixed, potassium sulfate and sodium chloridemixed, cuprous chloride, cuprous chloride and potassium chloride mixed,sodium chloride and potassium chloride and strontium chloride mixed,sodium dihydrogen phosphate, sodium tungstate combined with alkalichlorides or bromides, etc. Obviously many other combinations of theabove recited components may be found suitable. Since the softeningpoints and specific gravities of different glasses vary widely, it isimpossible to state the proportions of any of the above recited bathswhich will be suitable for all glasses, but for any individual glass theproper proportions can easily be determined by trial. It will further beapparent that some of the above recited heating bath compositions mayalso be found suitable for use as cooling baths.

Some salt mixtures which I have found to be particularly suitable aschilling baths both as regards wide range of temperature of operationand economy are as follows: sodium nitrate and potassium nitrate mixed,sodium nitrite and potassium nitrate mixed, sodium nitrate alone,potassium nitrate alone, ammonium acid sulfate, lithium nitrate andsodium nitrate and potassium nitrate and potassium nitrite mixed,potassium chloride and cuprous chloride mixed, zinc chloride alone,potassium chloride and zinc chloride mixed, potassium acid sulfate,sodium acid sulfate, sodium acid sulfate and potassium acid sulfate andothers. It will be apparent that various combinations of the above namedsalts other than those mentioned above may also be used.

In general, it has been found necessary to prevent the heating bath frombecoming alkaline through decomposition or volatilization, sinceotherwise many common types of glasses would thereby be attacked and thesurfaces thereof eched or the stability of the surface would be greatlydiminished. This I accomplish by introducing into the bath a smallamount of an acid oxide such as tungstic oxide, W03, or sand, SiOz, orvery small percentages of boric oxide, B203. I may also use ammoniumsalts, such as ammonium sulfate or ammonium chloride, which ondecomposition release an acid radical in the bath. Such additions arepreferably made in small increments from time to time as required. Inthe case of a bath composed of sodium chloride and potassium chloride,it was found that the introduction of sand or about 25% of boric oxideresulted in a better surface stability in the case of the abovementioned glass 13: of the Sullivan and Taylor patent than was obtainedwithout the addition of one or the other of these oxides.

I claim:

1. In a method of tempering glass, the steps of melting a salt whichtends to become alkaline on decomposition to form a bath, maintainingsaid bath at a temperature at which it will tend to decompose,maintaining in said bath a small quantity of a material which willneutralize the alkalinity of the bath, and immersing the glass in saidbath.

2. In a method of tempering glass, the steps of melting a salt whichtends to become alkaline on decomposition to form a bath, maintainingsaid bath at a temperature at which it will tend to decompose,maintaining in said bath a small quantity of boric oxide to neutralizethe alkalinity of the bath, and immersing the glass in said bath.

3. In a method of tempering glass, the steps of melting a salt whichtends to become alkaline on decomposition to form a bath, maintainingsaid bath at a temperature at which it will tend to decompose,maintaining in said bath a small quantity of ammonium sulfate toneutralize the alkality of the bath, and immersing the glass in saidbath.

4. In a method of tempering glass. the Steps of melting a salt whichtends to become alkaline on decomposition to form a bath, maintainingsaid bath at a temperature at which it will tend to decompose,maintaining in said bath a small quantity of tungstic oxide toneutralize the alkalinity of the 'bath, and immersing the glass in saidbath.

5. A substantially anhydrous fluid treating bath for glasswarecomprising a molten salt of an alkali metal and a substance which willreact with free alkali and prevent the bath from becoming alkaline.

6. A substantially anhydrous fluid treating bath for glasswarecomprising a molten salt of an alkali metal and boric oxide.

7. A substantially anhydrous fiuid treating bathfor glassware comprisinga molten salt of an alkali metal and ammonium sulfate.

8. A substantially anhydrous fluid treating bath for glasswarecomprising a molten salt of an alkali metal and containing tungsticoxide.

WILLIAM w. SHAVER.

